Blook oxygen saturation (SO.sub.2) is conventionally measured in vivo by inserting a fiber optic catheter into a blood vessel and detecting the relative reflectivity of the blood under red and infrared illumination. In one prior art device, an intensity ratio I=.lambda..sub.2 /.lambda..sub.1 was determined from a red intensity signal .lambda..sub.1 and in infrared intensity signal .lambda..sub.2. A linear transfer function of the form SO.sub.2 =BI+A was used to provide the saturation indication, with A being determined at the time of manufacture and B being obtained by adjustment of a calibration knob after intubation to match an in vitro analysis of a blood sample taken from the patient. This method provided accurate information only at the saturation level at which the sample was taken, and approximate information at all other levels.
Another prior art method (see U.S. Pat. No. 4,114,604) used three intensity signals .lambda..sub.1, .lambda..sub.2, and .lambda..sub.3 (typically on the order of 670, 700 and 800 nm respectively) from which two ratios I.sub.1 =.lambda..sub.1 /.lambda..sub.2 and I.sub.3 /.lambda..sub.2 were determined. The transfer function for the saturation indication was of the general form. ##EQU1## in which the A and B factors were selectively weighted so as to minimize the effect of varying physiological characteristics of the blood under test. Calibration in this method involved both additive and multiplicative aspects of the optical measurements. Nevertheless, the transfer function of this method produced still only an approximation of the real SO.sub.2 values, particularly at hematocrits differing substantially from a nominal hematocrit of about 35%.